nginx 1.25.1 deprecated the `listen ... http2` parameter in favor of a standalone `http2 on;` directive. Apps that ship a custom `nginx.conf.sigil` forked from an old default still hardcode `listen ... ssl http2;` and trigger `nginx: [warn]` lines from `nginx -t`. Surface a deprecation warning during `nginx_build_config` whenever the active template is app- or plugin-supplied and still contains the deprecated parameter, matching the existing warnings for `DOKKU_APP_LISTENERS`, `NGINX_SSL_PORT`, and `NGINX_PORT`, and point users at the `HTTP2_DIRECTIVE_SUPPORTED` variable already used by the default template.
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